Thanks for the suggestion. The JointJS core library, basically anything except of what is described here:
http://jointjs.com/rappid/docs (in other words, anything that is downloadable here:
http://jointjs.com/download) is licensed under the MPL v2 license. The rest is licensed under our own, commercial, license. Commercial support is indeed optional and covers bug fixes, general advise, help, online troubleshooting and more. We also do custom developments that are priced individually, depending on the project.